The gases in air, such as especially oxygen and nitrogen, are of great industrial importance, especially in the fields of papermaking or glassmaking.
One of the non-cryogenic techniques used for producing these gases is the technique called "PSA" (standing for Pressure Swing Adsorption), which technique covers not only the strictly speaking PSA processes, but also the similar processes such as the VSA (Vacuum Swing Adsorption) and the MPSA (Mixed Pressure Swing Adsorption) processes.
According to this PSA technique, when the gas mixture to be separated is air and the component to be recovered is oxygen, the oxygen is separated from the gas mixture by means of preferential adsorption of at least the nitrogen on a material preferentially adsorbing at least nitrogen and subjected to given pressure cycles in the separation zone.
The oxygen, not being adsorbed or only slightly so, is recovered as output from the separation zone; in general, this has a purity greater than 90%, or even greater than 93%.
More generally, a PSA process for the non-cryogenic separation of a gas mixture comprising a first component that is preferentially adsorbed on an adsorbent material and a second component less preferentially adsorbed on the adsorbent material than the first component, for the purpose of producing the second component, comprises, in a cyclic manner:
a step of preferential adsorption of at least the first component on the adsorbent material at an adsorption pressure called "high pressure", with recovery of at least some of the second component thus produced; PA1 a step of desorption of the first component, thus trapped by the adsorbent, at a desorption pressure below the adsorption pressure, called "low pressure"; PA1 a step of recompression of the separation zone comprising the adsorbent, by going from the low pressure to the high pressure. PA1 the average operating time (.mu.) of the valves is such that: EQU 0.5 s&lt;.mu.&lt;1.5 s, PA1 0.1 s&lt;x&lt;0.4 s, preferably 0.1 s&lt;x &lt;0.3 s; PA1 it includes at least 3 valves, preferably at least 5 valves; PA1 it includes at least two adsorbers, preferably two or three adsorbers; PA1 it is of the VSA type; PA1 it furthermore includes gas pipes; PA1 it furthermore includes a system for controlling the operating times of the valves; PA1 it furthermore includes a system for modifying the sequence of commands to operate the valves as a function of the operating times measured by the control system; PA1 it is of the type with radial circulation of the gas and/or employs one or more adsorbents, for example a multibed process; PA1 the gas stream to be separated comprises nitrogen and at least one less polar gas component, especially oxygen and/or hydrogen, and preferably the gas stream is air, the first gas component being nitrogen and the second gas component being oxygen, the air, in the context of the present invention, being the air contained inside a building or a heated or unheated enclosure, or is external air, that is to say under atmospheric conditions, taken as it is or possibly pretreated; PA1 the first gas component is nitrogen and the second gas component is oxygen and an oxygen-rich gas stream, that is to say generally comprising at least 90% oxygen, is produced; PA1 the high pressure for adsorption is between 10.sup.5 Pa and 10.sup.7 Pa, preferably of the order of 10.sup.5 Pa to 10.sup.6 Pa, and/or the low pressure of desorption is between 10.sup.4 Pa and 10.sup.6 Pa, preferably of the order of 10.sup.4 Pa to 10.sup.5 Pa; PA1 the feed temperature is between 10.degree. C. and 80.degree. C., preferably between 250.degree. C. and 60.degree. C.
However, it is known that the efficiency of separation of a gas mixture, such as air, depends on many parameters, especially the high pressure, the low pressure, the type of adsorbent material used and the affinity of the latter for the components to be separated, the composition of the gas mixture to be separated, the adsorption temperature of the mixture to be separated, the size of the adsorbent particles, the composition of these particles and the temperature gradient established inside the bed of adsorbent.
Currently, zeolites are the adsorbents most used in PSA processes. The zeolite particles usually contain monovalent, divalent and/or trivalent metal cations, for example cations of alkali metals, alkaline-earth metals, transition metals and/or lanthanides, these cations being incorporated during the synthesis of the zeolite particles and/or inserted subsequently using an ion-exchange technique, that is to say, in general, by bringing the unexchanged or raw zeolite particles into contact with a solution of one or more metal salts comprising the cation or cations to be incorporated into the zeolitic structure and subsequently recovering the particles of exchanged zeolite, that is to say of zeolite containing a given amount of metal cations. By way of example, mention may be made of zeolites of type X or LSX (Low Silica X) containing more than 80%, or even more than 90%, of metal cations such as, especially, lithium, calcium and/or zinc cations.
Such zeolites are especially described in documents EP-A-486,384, EP-A-606,848, EP-A-589,391, EP-A-589,406, EP-A-548,755, U.S. Pat. No. -A-5,268,023, EP-A-109,063 and EP-A-760,248.
In theory, a PSA, especially VSA, process cycle is composed only of a well-defined succession of steps which completely define the gas streams at each instant of the cycle.
However, in practice these gas streams are organized by a sequence of valve opening and closing operations which are not, of course, instantaneous.
Thus, there are transient states during which certain parts of the plant carrying out the process are inopportunely placed in communication with each other.
This may, for example, be an unintentional inflow of air at the start of the oxygen recompression phase, which inflow then necessarily affects the overall performance of the process.
It will be understood that those skilled in the art have always tried to limit the extent of these transient states by limiting as far as possible, on an industrial scale, the opening and closing times of the valves in PSA or VSA units to a value generally less than approximately 0.5 seconds.
This requirement appears to be all the more important the shorter the durations of the steps of the pressure cycle.
Moreover, as soon as the process requires the use of a PSA or VSA unit having more than one adsorber, that is to say two or three adsorbers for example, too wide a distribution in the opening/closing times in the population of valves of the PSA or VSA unit may very easily create large imbalances in the process and very significantly degrade the performance.
It is therefore usual, when attempting to minimize or alleviate these problems, to try to obtain actuation times which are as homogeneous as possible over all the valves of the PSA unit.
Like the requirement for rapid operation, this requirement is particularly critical in the case of short cycles, that is to say cycles having a period of less than 60 seconds for example.
Hitherto, attempts have been made to satisfy these two conditions by seeking valves approaching systems with instantaneous or almost-instantaneous operation as close as possible.
Thus, document U.S. Pat. No. 4,360,362 proposes a PSA system employing very rapid valves and using a single pneumatic control so as to erase any heterogeneity in operating time.
More generally, PSA or VSA units conventionally have a very narrow distribution in the operating times of the valves, around a very short average time, as shown schematically in FIG. 1 appended hereto, which shows a conventional distribution in the opening and closing times on a VSA unit.
In FIG. 1, it may be seen that, conventionally, the average opening/closing time of the valves is equal to approximately 0.3 s.+-.0.1 s.
In order to avoid such undesirable gas transfer or flow during the changes in step of a PSA cycle, valve control systems have already been proposed, especially by document U.S. Pat. No. 4,322,228 which imposes a delay in the opening/closing commands, so as to actuate the valve which has to close before the one which has to open, the intended objective being here to completely eliminate any transient transfer.
However, this approach considerably affects the choice of valve technology to be employed in the VSA units.
Consequently, certain documents, such as the documents U.S. Pat. No. 4,877,429, JP-A-05,192,526 or GB-A-2,190,014, propose the use of novel valves, particularly valves employing rotary technology, making it possible to speed up the operating times compared with those of valves corresponding to more proven technologies, such as butterfly valves.